The concept of Accumulated Heat Load Units (AHLU) has been developed to give some indication of the amount of heat that is accumulated by an animal when it is exposed to environmental conditions that are above its ability to maintain thermoneutral conditions.
For every hour that an animal is above its threshold HLI value, it will gain heat. This additional heat load accumulates over time and is reflected as an increase in body temperature.
It is a normal physiological response for animals to gain heat during the day and dissipate this accumulated heat to the environment at night. If the animal cannot dissipate this accumulated heat overnight, the animal carries a heat load into the following day. This makes the animal more susceptible to the effects of subsequent heat load.
There are three aspects in determining the potential for excessive heat load in feedlot cattle:
- There is a time effect, (i.e. the number of consecutive days that cattle are exposed to some accumulated heat load in excess of 25 units, even where they receive night time relief). This appears to be a major determinant in terms of susceptibility to the effects of subsequent excessive heat load events.
- There is an intensity effect. Feedlot cattle are highly vulnerable where there is a rapid onset (over 4 – 7 hours) of an extreme heat event where the HLI exceeds 100 units. Under these conditions cattle are highly vulnerable even where the Accumulated Heat Load Units are not excessive (e.g. 25 – 50 units).
- Feedlot cattle are very susceptible where they don’t have the opportunity to dissipate their accumulated heat load during the night or have the opportunity to recover sufficiently, and enter the next day with a pre-existing heat load. This is reflected as a ramping up of accumulated heat load units over subsequent days.
In order to calculate the AHLU, several parameters are required:
- The Heat Load Index (HLI)
- Upper limit of the Thermoneutral Zone (UL)
- Lower limit of the Thermoneutral Zone (LL)
- Interval (in hours) between successive HLI estimates (T)
- Stock characteristics
- Pen management practices
- Mitigation measures
The RAP calculator may be used to determine the upper limit based on the specific feedlot and stock characteristics.
The formula used to calculate the AHLU is as follows:
if HLI > ULExcess = HLI – ULif HLI < LLExcess = HLI – LLelseExcess = 0if Excess > 0New_AHLU = Old_AHLU + Excess * Tif Excess < 0New_AHLU = Old_AHLU + Excess * T / 2.0elseNew_AHLU = Old_AHLU
The upper limit of the thermalneutral zone can be found using the RAP calculator and is the HLI value at which stock begin to accumulate heat load.